Lunyu II. 11. (27)

To be able to teach others one must from his old stores be continually developing things new.
The Master said, "If a man keeps cherishing his old knowledge, so as continually to be acquiring new, he may be a teacher of others."

Legge II.11.

The Master said, 'A man is worthy of being a teacher who gets to know what is new by keeping fresh in his mind what he is already familiar with.'